In recent years, improvement of reliability is required for semiconductor integrated circuits (ICs) used in various fields. For an IC used for a product such as a liquid crystal river circuit for a car-mounted navigation or medical use, particularly high reliability is required. To achieve such high reliability of a product, endurance to external overvoltage (electrostatic discharge) should be increased. That is, an IC having high ESD endurance is required.
In order to increase the ESD endurance of an LSI (Large Scale Integrated circuit), a protection element against ESD (ESD protection element) is provided between an internal circuit and an outside (input/output pad) of the LSI chip. The ESD protection element changes a path of surge current generated due to the electrostatic discharge (ESD) to prevent the internal circuit of the LSI from being broken.
In general, as the ESD protection element, a MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor), an NPN bipolar transistor, or a thyristor is used. For example, the ESD protection element using the NPN bipolar transistor has been described in “ESD Protection Considerations In Advanced High-Voltage Technologies for Automotive” (non-patent literature 1), or IEEE Journal of Solid-State Circuits (Vol. 40, No. 8, P. 1751, August 2005 (non-patent literature 2)).
In the ESD protection element using an NPN bipolar transistor, a base voltage of the NPN bipolar transistor is pulled up by a high voltage due to ESD, to activate a bipolar transistor, and surge current flows through the NPN bipolar transistor. This can prevent the surge current from flowing into an internal circuit connected to a pad. Typically, junction breakdown generated due to the increase in the base voltage triggers the activation of the bipolar operation. However, if the junction breakdown locally occurs at a local point, current is concentrated on the local point, and therefore a variation occurs in current density of the surge current in a base width direction (W direction) to reduce the ESD endurance.
For this reason, the triggering of the bipolar operation is performed not by the breakdown of the NPN bipolar transistor itself but by an externally arranged trigger element, and as the result of this, the above-described degradation in the ESD endurance can be improved. For example, U.S. Pat. No. 5,850,095 (patent literature 1) describes an ESD protection element based on an NPN bipolar transistor provided with a trigger element.
FIG. 1 is a diagram illustrating a cross-sectional structure of the ESD protection element described in the patent literature 1. FIG. 2 is a circuit diagram illustrating an equivalent circuit of the ESD protection element described in the patent literature 1.
Referring to FIGS. 1 and 2, a structure and operation of the ESD protection element in the patent literature 1 will be described. Referring to FIG. 1, in the ESD protection element in the patent literature 1, a P-type substrate 101 (P-sub), and an N-type buried layer 102 (NBL) are formed in a Z-axis direction from a lower layer, and an N-type well 103 is formed on the N-type buried layer. A P-type well 104 is formed on the N-type well 103 to function as a base region. A heavily doped P-type diffusion layer (hereinafter to be referred to as a P+ base diffusion layer) 105 is formed in the P-type well 104, to function as a base terminal B10, and heavily doped N-type diffusion layers (hereinafter to be referred to as N+ emitter diffusion layers) 106 are formed in the P-type well 104, to function as an emitter terminal E10. Also, a heavily doped N-type diffusion layer (hereinafter to be referred to as an N+ collector diffusion layer) 107 is formed on the N-type buried layer 102, to function as a collector terminal C10.
The P+ base diffusion layer 105 is connected to a pad 100 through a trigger element (diode 200), and also grounded through a resistive element 300 (R10). The N+ emitter diffusion layer 106 is grounded. The N+ collector diffusion layer 107 is connected to the pad 100. The pad 100 is connected to an internal circuit (not shown). The trigger element is the diode 200, and an anode of the diode 200 is connected to the P+ base diffusion layer 105 and a cathode thereof is connected to the pad 100 and the N+ collector diffusion layer 107. Based on such a structure, the ESD protection element according to a conventional technique is represented by the equivalent circuit illustrated in FIG. 2.
Referring to FIG. 2, if a voltage applied to the pad 100 due to ESD exceeds a breakdown voltage of the diode 200, a current flows from the base terminal B10 toward GND. At this time, the resistance element 300 provided between the base terminal B10 and the GND causes a voltage at the base terminal B10 (base voltage) to increase, and surge current due to the ESD starts to flow between the collector terminal C10 and the emitter terminal E10. That is, the bipolar operation is started in response to triggering by the trigger element, and the surge current can be prevented from flowing into the internal circuit.